For Williamson, this was his second crown of the season. He won earlier this year at the California Classic.
'Coming out of college, I knew I had a chance to have a good career,' stated Williamson. 'It hasn't been until the last year that I have been able to find that kick. It's been quite a year.'
Andy Doeden, who shared the lead entering the final round with Williamson, managed just a one-over 73 to end at minus-seven. He finished alongside Jason Hartwick, who led after each of the first two rounds.
Darren Griff took fourth place at six-under-par 282 after closing with a 71 at Hazelmere Golf Club.
Williamson fell out of the lead early on with a double-bogey on the first. However, he quickly atoned for those errors with birdies on two and three.
The 26-year-old bogeyed the seventh. Around the turn, Williamson picked up birdies on 10 and 13. After he bogeyed the 15th, Williamson parred the final three holes to take his second title this season.
'Yeah, my heart did jump a little,' Williamson said after his putt on 18 bounced off the back of the cup and into the hole. 'It is a little bit of a relief to get this one. It was tough out there. It was a championship golf course that gave us a real test. I've played better and scored worse, so I'll take this one.'
Hartwick birdied the third, but gave that stroke right back with a bogey on four. He dropped another stroke on seven. Hartwick finally atoned for that error with a birdie on 14. He parred out from there to end one back.
Doeden bogeyed one and two, but then birdied the third. He slipped to another bogey on four, but got that back with his fourth birdie in four days at the seventh.
The 26-year-old stumbled to a double-bogey on eight, but got those strokes back with birdies on 11 and 17. It was not enough, though, as he finished at minus-seven.
Brock Mackenzie had the best final round as he posted a five-under 67. That helped him jump into a share of fifth place at minus-five, where he was tied with Chad Spencer (69) and Steve Friesen (72).